This invention relates to a composite material having corrosion resistance at high temperatures and method of manufacture and, more particularly, to the material and method for producing a metal foil material for use in catalytic converters.
As well known, exhaust gases discharged from motor vehicles may contain halogen gases, halogen compounds and lead compounds, for example, Cl.sub.2, Br.sub.2, PbCl.sub.2, C.sub.2 H.sub.2 Cl.sub.2, C.sub.2 H.sub.2 Br.sub.2 etc., besides unburnt noxious gases including carbon monoxide, hydrocarbon and the like, and components or parts made of ferrous base alloy material for exhaust system of the motor vehicles or the like, for example, heat exchangers, air ducts, containers, etc., tend to be subjected to corrosion by the noxious compounds as described above. Moreover, halogen compounds (e.g. salt) employed for preventing freezing during cold seasons are liable to enter these components of ferrous base alloy material, which are then corroded by the atmosphere containing halogen gas produced when the halogen compounds are decomposed at high temperatures.
It has been known to use metal foil materials as substrates with an appropriate catalyst coating in place of ceramic material substrates. Such metal foil material has been made by ingot metallurgy from steel sheets containing aluminum and also chromium in order to have the desired corrosion resistance at high temperature. These FeCrAl alloys, however, are difficult to produce by conventional rolling and annealing processes. To overcome the processing difficulties, it has been suggested, as in EP application 91115501.8, to produce the foil by a rapid solidification processing method; but such processing is expensive and requires very precise controls. It has also been suggested to dip the stainless steel in a molten bath of aluminum or aluminum alloy to apply melt-plating on the surface of the stainless steel (U.S. Pat. Nos. 3,907,611, 3,394,659 and 4,079,157). This stainless steel with the aluminum is then subjected to a heat treatment to form an alloy layer having Fe and Al as the main components. Still further, surface layers of aluminum in binder materials, as described in U.S. Pat. No. 4,228,203, have also been suggested. However, in all of these applications the control of the processing parameters is complex and expensive; and the final foil has not proven, in many cases, to have the desired corrosion/oxidation resistance at elevated temperatures.
Still another approach is to manufacture the catalytic converter substrate material by using a metallurgically bonded composite material with layers of ferritic stainless steel and aluminum as described in U.S. Pat. No. 5,366,139 owned by the assignee of this instant application. This substrate material has proven to be very useful but in certain applications improved formability and improved dimensional stability is desired